linux-kernel - Re: [PATCH] drm/cirrus: rewrite and modernize driver.

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Date:   Wed, 03 Apr 2019 11:12:05 -0400
From:   Adam Jackson <ajax@...hat.com>
To:     Gerd Hoffmann <kraxel@...hat.com>, dri-devel@...ts.freedesktop.org
Cc:     David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVER FOR QEMU'S CIRRUS DEVICE" 
        <virtualization@...ts.linux-foundation.org>,
        Dave Airlie <airlied@...hat.com>
Subject: Re: [PATCH] drm/cirrus: rewrite and modernize driver.

On Wed, 2019-04-03 at 09:23 +0200, Gerd Hoffmann wrote:

>  - Only DRM_FORMAT_RGB565 (depth 16) is supported.  The old driver does
>    that too by default.  There was a module parameter which enables 24/32
>    bpp support and disables higher resolutions (due to cirrus hardware
>    constrains).  That parameter wasn't reimplemented.

One slightly annoying aspect of this (well, initially of the patch to
clamp the default to 16bpp, but this too) is that we only have a way to
ask the driver which format it prefers, not which ones it supports at
all. For X's modesetting driver (and yes some of this is because X is
awful) this creates the following failure mode:

1: user sets up xorg.conf for depth 24
2: user upgrades kernel, reboots
3: X driver detects that depth 16 is preferred, but
4: X core respects user's xorg.conf and tries depth 24, which
5: throws -EINVAL and X won't start.

Possibly X should work around this by transparently setting up a shadow
framebuffer at the user's requested depth. The problem there is, if 565
is preferred but 8888 works, you're adding a format-conversion blit in
the middle for no reason. If I could ask the kernel for the entire list
of supported formats, I could only set up the shadow if it was
necessary.

- ajax